The viruses causing influenza have been arbitrarily named as influenza type A, B, and C. These types define antigenically distinct viruses. Each type has several distinct subtypes. Viruses within one type are genetically compatible in the sense that cells infected with two different subtypes can assemble mixed viruses containing components from both subtypes. Influenza viruses are classified as orthomyxoviruses. The viruses form particles of between 80 and 120 nm in diameter. Influenza viruses are enveloped viruses. i.e., their outer surface is derived from host cell membrane. Inserted in and protruding from the envelope are two major viral-encoded proteins, hemagglutinin (HA) and neuraminidase (NA). Influenza viruses are negative-stranded RNA viruses, containing a genome made up of 8 RNA segments of non-messenger RNA polarity. The genomic RNA segments are assembled in RNP complexes with virus-encoded nuclear protein (NP). Following infection of a host cell, genomic RNA segments are first transcribed into RNAs with messenger RNA polarity which are later reverse-transcribed to produce genomic RNA. The transcriptase activities responsible for these steps lacks proof-reading capability. Mistakes that are made during transcription and reverse transcription are therefore not repaired, resulting in a high frequency of mutation of the viral genome. While all viral genes are subject to the same mutational process, genes for external proteins HA and NA are particularly subject to strong selection processes that drive their evolution towards mutant forms that escape immune detection in their hosts. Hosts include not only humans but also animals such as chicken, turkey, swine and horse.
Influenza has traditionally been one of the leading causes of human death. The clinical signs of the influenza are variable, ranging from asymptomatic to fatal infection. Typically, onset of illness is rapid and prostrating, and is almost invariably attended by cough, malaise, headache, and myalgia. Coryza, sore throat, and, less commonly, substernal pain also indicate that the primary site of infection is the respiratory system. Typically, however, fever and systemic symptoms predominate. Recovery typically is rapid. The severity of the disease is largely host-dependent and relates to age, physiological state and prior immunization by infection or immunization. A severe complication is pneumonia. Compromised individuals are prone to suffer secondary infections with bacterial pathogens that cause pneumonia. Most patients who die following influenza die with bacterial pneumonia. Minor antigenic variations in influenza virus types A and B occur yearly, causing regional epidemics. The yearly death rate from such yearly epidemics may approach 20,000 in the U.S. alone. At variable intervals between 10 and 30 years, global pandemics occur with death tolls far exceeding that of yearly epidemics. These pandemics are probably caused by genetic reassortment of components from human and animal influenza A viruses, resulting in new virus with a surface structure total alien to human experience. The death toll of the 1918-19 pandemic killed about 500,000 Americans. (As a general reference: Joshua Lederberg, Encyclopedia of Microbiology, 2(D-L):505-520, Academic Press Inc., San Diego, Calif. 92101 (1992).
Presently licensed vaccines include inactivated purified virus. The vaccines are trivalent and include representative strains of the two prevalent A subtypes, H3N2 and H1Ni, and a single type B strain. Attenuated live virus vaccines have also been used with some success, particularly in the previous Soviet Union. Subunit vaccines have been developed containing HA and NA (split flu vaccine; Connaught Lab.). These vaccines are not completely effective in providing protective immunity. It is generally accepted that influenza vaccines generate protective immunity mainly by means of inducing antibody responses to the viral surface proteins HA and NA. This may explain why the vaccines are only incompletely effective; they are susceptible to continuous antigenic variation in these surface proteins.
The search for differences between tumor cells and normal cells has led to the isolation and characterization of a number of so-called tumor-associated antigens (Henderson, R. A. and Finn, O. J. Advances in Immunology, 62:217-256 (1996)). These antigens are expressed by tumor cells but not at all or at least not in large amounts in fully differentiated cells. The sequences encoding these tumor antigens are either virus-derived or are normally present in the genome of the host. An example of a virus-derived tumor-associated antigen is the human papillomavirus transforming protein E7 present in most human cervical tumors. A typical host genome-derived tumor-associated antigen is gp 100, also referred to as pMel-17, that is expressed in many human melanomas. While tumor-associated antigens are known to induce a host immune response, the response is typically insufficient to be therapeutically effective. There is a need for approaches to stimulate this response.
Using monospecific cytotoxic T lymphocyte (CTL) clones, the expression of at least five tumor-associated antigens, termed A, B, C, D and E, has been identified in mouse P815 mastocytoma tumor cells. One of these antigens, termed P1A, expresses a single epitope that is recognized by CTL clones. Using a molecular approach, the gene for P1A was cloned and was found to be a nonmutated gene present in normal cells but transcribed and translated only in transformed cells (Van den Eynde et al., J. Exp. Med., 173:1373 (1991)). Further, by examination of variants of P815 cells that had lost P1A antigen expression, it was possible to identify the sequence of the MHC class I (Ld)-restricted minimal CTL epitope of P1A (Lethe, et al., Eur. J. Immunol. 22:2283 (1992)).
Thus, a need exists for more effective vaccines against antigens associated with viruses and tumors.